Article pubs.acs.org/Langmuir
Alcohol Dispersions of Calcium Hydroxide Nanoparticles for Stone Conservation Carlos Rodriguez-Navarro,* Amelia Suzuki, and Encarnacion Ruiz-Agudo Department of Mineralogy and Petrology, University of Granada, Fuentenueva s/n, 18002 Granada, Spain ABSTRACT: Alcohol dispersions of Ca(OH)2 nanoparticles, the so-called nanolimes, are emerging as an effective conservation material for the consolidation of stone, mortars, and plasters present in old masonry and/ or mural paintings. To better understand how this treatment operates, to optimize its performance and broaden its applications, here we study the nano and microstructural characteristics, carbonation behavior, and consolidation efficacy of colloidal alcohol dispersions of Ca(OH)2 nanoparticles produced by both homogeneous (commercial nanolime) and heterogeneous phase synthesis (aged slaked lime and carbide lime putties). We observe that the alcohol not only provides a high colloidal stability to Ca(OH)2 particles, but also affects the kinetics of carbonation and CaCO3 polymorph selection. This is due to the pseudomorphic replacement of Ca(OH)2 particles by calcium alkoxides upon reaction with ethanol or 2propanol. The extent of this replacement reaction depends on Ca(OH)2 size and time. Hydrolysis of alkoxides speeds up the carbonation process and increases the CaCO3 yield. The higher degree of transformation into calcium alkoxide of both the commercial nanolime and the carbide lime fosters metastable vaterite formation, while calcite precipitation is promoted upon carbonation of the aged slaked lime due its lower reactivity, which limits calcium alkoxide formation. A higher consolidation efficacy in terms of strength gain of treated porous stone is achieved in the latter case, despite the fact that the carbonation is much faster and reaches a higher yield in the former ones. Formation of alkoxides, which has been neglected in previous studies, needs to be considered when applying nanolime treatments. These results show that the use Ca(OH)2 nanoparticle dispersions prepared with either aged slaked lime or carbide lime putties is an economical and effective conservation alternative to commercial nanolimes produced by homogeneous phase synthesis. Ultimately, this study contributes to show that nanotechnology can help saving the built and sculptural heritage.
1. INTRODUCTION The survival of mankind’s cultural heritage is challenged by chemical, physical, and biological weathering phenomena.1 In an attempt to halt and/or mitigate the degradation of materials used in the built and sculptural heritage, several conservation treatments have been developed and applied.1 Until recently, most widely used treatments for the protection and consolidation of damaged stone, mortars and plasters, and mural paintings involved the application of synthetic polymers such as acrylic, epoxy, or (poly)vinyl resins, as well as their copolymers. However, polymer protectives and consolidants are generally incompatible (physically and chemically) with the inorganic substrate they are applied to, thus fostering damage.3 For instance, Giorgi et al.3 report on the deleterious effects of a copolymer treatment (vinyl-acetate/n-butyl-acrylate) applied on degraded mural paintings in the Maya site of Mayapan (Yucatan, Mexico). The formation of an impervious organic surface coating led to enhanced salt damage and the loss of painted areas. The physicochemical incompatibility and poor aging of most polymer-based conservation treatments have prompted the resurgence of inorganic conservation materials.4 One of their principal advantages is that they can be chemically and structurally similar to the inorganic substrate they are applied © XXXX American Chemical Society
to, thereby ensuring a high compatibility. There is a long tradition on the use of inorganic compounds for the protection and consolidation of building materials such as stone. For instance, limewater, i.e., a calcium hydroxide saturated solution, has been applied to consolidate degraded porous substrates, especially those of carbonatic nature (e.g., limestones or lime mortars).4 During carbonation, atmospheric CO2 dissolves in the calcium hydroxide solution, leading to precipitation of calcium carbonate according to the (overall) reaction: Ca(OH)2 + CO2 = CaCO3 + H2O. The newly formed CaCO3 can act as a cement, binding loose grains and/or filling cracks. However, the limewater method does not seem to be very effective,1,4 principally because of the limited solubility of Ca(OH)2 in water (1.7 g L−1 at 20 °C) which precludes the introduction of sufficient amounts of consolidant into the porous system of the treated material.5 The use of large quantities of water is also an important risk:6 e.g., potential substrate dissolution, clay-swelling, mobilization of salts (fostering salt weathering), or freeze−thaw damage. Received: May 9, 2013 Revised: July 1, 2013
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dx.doi.org/10.1021/la4017728 | Langmuir XXXX, XXX, XXX−XXX
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parameters affect the slaking process and the properties of the resulting hydroxide crystals, such as CaO reactivity, oxide/water ratio, slaking water T, local overheating, vapor- vs liquid-phase hydration, and presence of additives (e.g., alcohols).18,19 In addition, aging of slaked lime putty leads to a reduction of portlandite crystals size and the overdevelopment of (0001) basal faces. These nanoparticles have a high surface/volume ratio, which enhances their reactivity (e.g., dissolution kinetics and carbonation rate), and their dispersions show a high colloidal stability and improved rheological properties.19,20 Slaked lime is typically made up of primary Ca(OH)2 nanoparticles 20 to 300 nm in size.18,19 Aggregation leads to significant size polydispersity due to the formation of micrometer-sized clusters. Our previous studies suggest that the high yield, ease of preparation, and nanosized nature of aged slaked lime putty make it a good candidate for stone consolidation using the nanolime methodology developed by Baglioni’s group.5 Heterogeneous synthesis of portlandite nanoparticles also takes place during several industrial processes, including paper and acetylene (C2H2) production. In the latter case, the socalled carbide lime or lime sludge is produced.21,22 Carbide lime (CL) forms as a byproduct of the hydrolysis of Ca2C in water leading to the generation of C2H2 and CaO. The newly formed CaO rapidly hydrates, yielding a Ca(OH)2 slurry. CL is an impure hydroxide which contains residues of both inorganic carbon (unreacted carbon) and organic carbon (hydrocarbons), along with traces of sulfur compounds (H2S) and aluminosilicates.21,22 CL may also include trace amounts of metals such as Fe, Pb, Cu, Cd, Ni, Cd, and Zn.22 These residues originate from impurities present in the coal and limestone used for calcium carbide synthesis, which takes place at a high T (2300 °C) according to the reaction CaO + 3C = CaC2 + CO (CO + 1/2O2 = CO2). Although CL has found some applications in the cement industry, soil stabilization, agriculture, asphaltic paving mixes, and sewage/water treatment,21,22 the presence of impurities has precluded/limited the use of this waste material for the numerous industrial and technical applications of Ca(OH)2.21,22 In fact, this waste lime is typically discarded and accumulated in ponds.22 This procedure is an economical burden and possesses some environmental risks (e.g., accidental spilling and/or groundwater contamination). We have recently patented a purification process for CL involving the oxidation of the organic carbon and sulfur compounds, the precipitation of the latter as insoluble barite (BaSO4) following addition of Ba(OH)2, and immobilization (coprecipitation) of heavy metals in the insoluble barite which is subsequently eliminated by gravity separation.22 This purified material, with commercial name Geosilex, is an aqueous dispersion of Ca(OH)2 nanoparticles (carbide lime putty) which typically includes a dispersant such as lignosulfonate (0.001−0.01 wt %), added to avoid bleeding and to foster disaggregation of Ca(OH)2 particles. Its physicochemical characteristics are very similar to those of aged slaked lime putty. In addition to its nanophase nature, the purified CL has the principal advantage of being a waste product whose recycling is environmentally friendly and of economic significance. Currently, this purified CL is finding applications in the construction industry as a binder with a net positive CO2-capture balance. Due to its nanophase character, carbide lime putty should also be a good alternative for the preparation of nanolime dispersions for conservation applications.
To overcome the limitations of the limewater treatment, Giorgi et al.5 proposed the use of aliphatic alcohol dispersions of portlandite (Ca(OH)2) particles for the protection and consolidation of limestone and, particularly, lime-based plasters with painted surfaces. The authors reported that aqueous suspensions of portlandite particles were kinetically unstable and settled rapidly. In addition, the higher contact angle and viscosity of water, and the resulting limited sorptivity,7 precluded a sufficiently high penetration of the suspended particles once applied on a porous substrate. These effects resulted in unacceptable surface white glazing. By using shortchain alcohols (e.g., 1-propanol), Giorgi et al.5 obtained highly stable dispersions of Ca(OH)2 particles which enabled the consolidation of lime mortars and mural paintings. In this pioneering study, Ca(OH)2 dispersions were prepared using dry hydrated lime (powder) or slaked lime (lime putty) (i.e., produced via heterogeneous synthesis, see below) with micrometer-sized particles, presumably aggregates of submicrometer primary particles. Afterward, colloidal Ca(OH)2 nanoparticles were homogeneously synthesized in diols,8 aqueous solutions,9 and water-in-oil microemulsions.10 This bottom-up synthesis resulted in nanophase portlandite crystals, known as nanolimes. It was reported that nanolimes have superior properties, including a higher reactivity, a higher penetration efficiency, and a higher kinetic stability when dispersed in aliphatic alcohols such as ethanol, 1-propanol, or 2propanol.11,12 Recent studies have focused on the analysis of the carbonation, consolidation efficacy, and improvement in the homogeneous synthesis of nanolimes applied to the consolidation of stone, lime mortars, and mural painting, as well as for paper, canvas, and wood deacidification.2,12−14 Currently, the standard route for the preparation of nanolimes is homogeneous phase synthesis in an aqueous solution. However, this route has some drawbacks.14 First, the synthesis has to be performed at T ≥ 90 °C and is timeconsuming as it involves the dropwise addition of NaOH solution to an aqueous solution of CaCl2,15 or Ca(NO3)2,16 under vigorous stirring. Second, the resulting NaCl (or NaNO3) byproduct has to be eliminated. For this task, after discarding the supernatant, the remaining suspension is washed with deionized water up to 5 times.9 Finally, the aqueous dispersion of portlandite particles is dispersed in alcohol (typically, ethanol or 2-propanol) at a desired concentration (e.g., 5 g/L). All in all, this synthesis route is costly and the yield is low. But there is an additional problem: during each washing/rinsing performed to eliminate residual Na salt, a fraction of Ca(OH)2 particles will be dissolved. Of all Ca(OH)2 particles, those with size
